FFL is a new type of light source. It possesses the advantages of containing no mercury and relatively having low-profile, long life-span, better uniform brightness and broader operating temperature range etc. It is mainly applied to the backlight systems, the flat lighting and outdoors etc. The FFL is one kind of DBDL. During the driving process of the FFL, there are electrical charges accumulated on an isolation layer to form the reverse potential (the wall voltage) against the driving voltage so as to stop the continuation of the discharging due to the existence of the isolation layer. Therefore, the polarities of the driving voltage have to be changed continuously to maintain the charging and discharging.
Under the aforementioned circumstances and among all the tryouts for driving the FFL, the half-bridge driver circuit is relatively quite effective in driving the FFL. The half-bridge driver circuit includes two switches (SA and SB) and generates stable square wave outputs via alternately turning on, turning off and continuously switching of SA and SB (as shown in FIG. 1). The square wave voltage is amplified via a transformer and is converted into a high voltage square wave for applying to the FFL so as to effectively drive the FFL. In which, Tdimming is the dimming period, TON is the turn-on time of the burst mode, and TOFF is the turn-off time of the burst mode.
This kind of stable square wave is ideal for the stable operation of the FFL, but it can not effectively handle the problems such as the startup and the repeated re-lighting in the burst mode dimming. It is because the ionization level in the plasma is relatively lower then, and the wall voltage existed on the isolation layer is not high enough relative to that in stable operation. This causes the problem of relatively harder startup and the light blinking problem in the dimming operations. The light blinking problem becomes serious especially when the dimming value is relatively lower, which makes the dimming value of the FFL has to be higher than around 20%.
The relatively lower dimming value makes the turn-off time of the burst mode relatively longer. The longer off time makes the combination of the positive ions and the negative ions of the plasma more complete, which makes the re-lighting problem of the FFL become even more difficult. Under such a circumstance, the FFL usually needs five to six driving periods to be lit up totally. When the FFL is not totally lit up under such a driving situation, the light blinking phenomenon of the FFL does occur, and the light blinking problem will get worse when the dimming value decrease.
As shown in the experiment of FIG. 2, the turn-on time of the burst mode is set at a fixed value (about 260 ns), and the turn-off time of the burst mode is set between 34 μs˜2.5 ms. As shown by the results of this experiment, the shortened off time can not speed up the re-startup in the dimming operations. Therefore, decreasing the turn-off time of the dimming operation can not solve the re-lighting problem during the dimming. Namely, the re-lighting problem always occur no matter what dimming value is, human eyes can not identify it only at the time dimming value is relatively higher. That is to say, the higher the ratio of the not totally lit up part over the total turn-on time of the burst mode is, the more obvious the blinking light to the human eyes is. Otherwise, the re-lighting problem is not so obvious to the human eyes. According to the above-mentioned descriptions, increasing the turn-on time of the burst mode dimming and decreasing the time from the re-startup to the total light-up would diminish the occurrence of the blinking light.
Increasing the turn-on time of the burst mode at the same dimming value would relatively increase the dimming time period, that is to say the dimming frequency would be decreased. Since the dimming frequency can not be too low in real operation, this kind of dimming has a relatively larger limitation, a relatively smaller dimming range. A method, which employs the burst mode dimming and could decrease the blinking light, is needed.
Besides, the energy stored on the isolation layer of the FFL can not be properly released, and will generate the voltage oscillation waves having a frequency lower than the PWM driving frequency, and the energy stored on the FFL is transformed into heat via oscillations and being dissipated when the turn-off time of the burst mode arrives during the burst mode dimming as shown in FIG. 3. This voltage oscillation generates a relatively larger stress on the transformer, especially when the dimming value is relatively higher (the turn-off time of the burst mode is relatively shorter then). When the next driving period comes, and the voltage oscillation is not diminished yet, which will cause an even larger stress to the transformer, and result in a magnetic bias phenomenon. In the present invention, a method of releasing energy at the end of each turn-on time of the burst mode is employed to let the FFL generate self-discharging. In this way, not only the lighting effect of the FFL is increased, but also certain wall voltage is remain on the FFL after the discharging is ended, which will facilitate the next startup.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a DBDL system and the driving method thereof having a relatively better performance in startup and re-startup of dimming.